Many electronic devices operate off a periodic clock signal to synchronize the transmission of data between electronic components within the device. Periodic clock signals can be provided by an oscillator, such as voltage controlled oscillator (VCO) or digitally controlled oscillator (DCO). Periodic clock signals are also used in radio devices and wireless devices to generate specified frequencies that can be used for a variety of purposes including upconversion, downconversion, and transmission on a carrier frequency. A wireless communication device may be allocated a specified range of frequency bands in which data is transmitted. A VCO/DCO can be used in a phase-locked loop (PLL) to generate various frequency bands. A time-to-digital converter (TDC) can be used to measure the VCO/DCO instantaneous phase with respect to a clean reference clock in a phase-locked loop (PLL) in order to lock the frequency of the VCO/DCO to specified frequency.
A wireless communication device (e.g., user equipment (UE)) for emerging gigabit per second wireless standards can require local oscillator (LO) signals with very good spectral purity. Digital fractional-n PLLs can be an attractive choice for LO generation in high performance radio transceivers on account of the small area, ease of porting, PVT (process, voltage and temperature) insensitive loop dynamics and scan programmability/re-configurability. The resolution and linearity of the time-digital converter (TDC) can limit the spectral purity achievable with a digital fractional-n PLL. As an example, a fractional-n PLL for the 5 GHz band can require the TDC to span a dynamic range of two hundred picoseconds (ps). At the same time, the TDC resolution and non-linearity can be required to be better than 1 ps in order to meet a phase noise and spur requirements of devices using gigabits per second (Gbps) wireless standards. Gbps wireless standards can include the third generation partnership project (3GPP) long term evolution (LTE), the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard (e.g., 802.16e, 802.16m), which is commonly known to industry groups as WiMAX (Worldwide interoperability for Microwave Access), and the IEEE 802.11 standard (e.g., 802.11ac, 802.11ad), which is commonly known to industry groups as WiFi.
A conventional digital PLL achieves a several pico-second TDC resolution through the use of techniques such as delay Verniers or delay interpolation. Mismatch and variations along the TDC delay line can introduce non-linearity and non-monotonicity in the TDC behavior, further degrading the PLL performance. The TDC delay cells can be sized up to reduce mismatch and variations. Consequently, the power dissipation can increase quadratically with resolution, rendering the design impractical for higher resolution TDCs with sub-picosecond resolution.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.